Interpretive Summary: The lack of weed control is often the most troublesome pest problem faced by organic producers. One potential organic herbicide is vinegar. There is a need to determine how much vinegar to apply, at what concentrations, and if additives will increase the weed control ability of the vinegar. Typically, household vinegar contains 5% acetic acid. Although acetic acid is naturally occurring, care must be taken when handling vinegar, especially as the acetic acid concentration increases above the typical 5%. Vinegars with acetic acid concentrations of 11% or greater can burn the skin and cause serious to severe eye injury, including blindness. Field research was conducted in southeast Oklahoma (Atoka County, Lane, OK) at the USDA, ARS, South Central Agricultural Research Laboratory to determine the effect of acetic acid concentrations, application volumes, and additives on weed control. One month prior to applying the weed control treatments, the land was cultivated to kill the existing weeds and to provide a uniform seedbed for new weed growth. The research involved 20 weed control treatments with 4 replications. The research design included vinegar at three acetic acid concentrations (0, 5 and 20 %), two sprayer application volumes [20 and 100 gpa, three additive comparisons (none, orange oil, and canola oil), and two weedy-checks. Visual weed cover and control ratings were collected 7 days after treatment on July 23, 2005. The experiment had very high weed densities with multiple species of grass and broadleaf weeds. Most weeds were very small with only 1 or 2 leaves. Total weed control ranged from 0 control when no vinegar was used to 48% control when 20% acetic acid was applied at 100 gpa with canola oil. Vinegar was less effective in controlling grasses than in the control of broadleafs. Optimum total grass and crabgrass weed control occurred with 20% acetic acid applied at 100 gpa, resulting in weed control that ranged from 28 to 45%. Broadleaf (total, carpetweed, tumble pigweed, and spiny amaranth) control was 100% for plots receiving either 5% acetic acid applied at 100 gpa or 20% acetic acid applied at 20 or 100 gpa. In this research, carpetweed was the most susceptible to vinegar applications. Carpetweed was 100% controlled with either 5 or 20% acetic acid, whether applied at 20 or 100 gpa. The use of additives didn't seem to improve the weed control in general. When averaged across application volumes (20 and 100 gpa) and adjuvants (none, orange oil, and canola oil), weed control increased for all species as acetic acid concentrations increased from 0 to 20%. In the same respect, when averaged across acetic acid concentrations and adjuvants, weed control increased as application volumes increased from 20 to 100 gpa. This research provides important guidelines for organic producers concerning the use of vinegar for organic weed control, especially the impact of acetic acid concentration and application volume on weed control.

Technical Abstract:
Acetic acid acts as a contact herbicide, injuring and killing plants by first destroying the cell membranes, which causes the rapid desiccation of the plant tissues. Vinegars with acetic acid concentrations of 11% or greater can burn the skin and cause serious to severe eye injury, including blindness. Field research was conducted in southeast Oklahoma (Atoka County, Lane, OK) to determine the effect of acetic acid concentrations, application volumes, and adjuvants on weed control efficacy. One month prior to applying the weed control treatments, the land was cultivated in order to kill the existing weeds and to provide a uniform seedbed for new weed growth. The research involved 20 weed control treatments with 4 replications. The factorial experimental design included vinegar at three acetic acid concentrations (0, 5 and 20 %), two sprayer application volumes [20 and 100 gpa (187 and 935 L/ha)], three adjuvants (none, orange oil, and canola oil), and two weedy-checks. The canola oil and orange oil were mixed at a 0.25% volume/volume (v/v), depending on the application volume (20 or 100 gpa). A 0.025% v/v of liquid dish soap was added to the treatments containing canola oil to reduce the surface tension of the oil and thus allow the canola oil to go into suspension. All herbicide treatments were applied on July 16, 2005. Visual weed cover and control ratings were collected 7 days after treatment on July 23, 2005. The experiment had very high weed densities with multiple species of grass and broadleaf weeds. Large crabgrass was the dominant weed, covering 85% of the weedy-check. At the time of spraying, large crabgrass plants averaged 1 or 2 leaves; however, the plots did include a few larger crabgrass plants that had regrown from the earlier tillage operation. Carpetweeds averaged 1 inch wide with 4 or 5 leaves, while spiny amaranth and tumble pigweed seedlings had only 2 or 3 leaves. Total weed control ranged from 0 control when no vinegar was used to 48% control when 20% acetic acid was applied at 100 gpa with canola oil. Vinegar was less effective in controlling grasses than in the control of broadleafs. Optimum total grass and crabgrass weed control occurred with 20% acetic acid applied at 100 gpa, resulting in weed control that ranged from 28 to 45%. Broadleaf (total, carpetweed, tumble pigweed, and spiny amaranth) control was 100% for plots receiving either 5% acetic acid applied at 100 gpa or 20% acetic acid applied at 20 or 100 gpa. In this research, carpetweed was the most susceptible to vinegar applications. Carpetweed was 100% controlled with either 5 or 20% acetic acid, whether applied at 20 or 100 gpa. Individual comparisons among adjuvants within acetic acid concentrations and application volumes showed little or no advantage to adding either orange oil or canola oil to vinegar spray solutions. When averaged across application volumes (20 and 100 gpa) and adjuvants (none, orange oil, and canola oil), weed control increased for all species as acetic acid concentrations increased from 0 to 20%. In the same respect, when averaged across acetic acid concentrations and adjuvants, weed control increased as application volumes increased from 20 to 100 gpa. There were few differences among the adjuvants when their responses were averaged across acetic acid concentrations and application volumes.